Yesterday I moved a customer's server to new disks. To get some extra features like snapshotting I opted to convert the current static disk layout to LVM on top of MDADM. As a bonus I did the entire sync online to a new disk connected to my laptop which acted as a degraded mirror.

Yes I know GPT is the way to go but I started this move at 7 PM to minimise business impact and since the new disks are still only 500G I went for MBR which was already in place. For GPT "RAID" check this answer

This tutorial can also be used to move a non-RAID server to a RAID setup or to move a server to a new machine that will have MDADM RAID.

First create the partition table

parted /dev/sdz 
mklabel msdos
mkpart primary ext2 0% 100%
set 1 lvm on
quit

Create the MDADM mirror and LVM group + partitions (I will only use 2 in this tutorial but you can use this as a guide to get a better solution for a professional environment)

mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdz1 missing
pvcreate /dev/md0
vgcreate lvm /dev/md0
lvcreate -L 500M lvm -n boot
lvcreate -L 20G lvm -n root
mkfs.ext4 /dev/mapper/lvm-boot
mkfs.ext4 /dev/mapper/lvm-root

 Next mount the new partitions and start cloning the old system

mount /dev/mapper/lvm-root /mnt
mkdir /mnt/boot
mount /dev/mapper/lvm-boot /mnt/boot
rsync -aAXv --exclude={"/dev/*","/proc/*","/sys/*","/tmp/*","/run/*","/mnt/*","/media/*","/lost+found"} user@src /mnt/
*here you should stop all services on the src and rerun the rsync command for a final sync*

Next we will set up the bootloader and adapt the system files

for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo cp /etc/resolv.conf /mnt/etc/resolv.conf
sudo chroot /mnt
blkid
*get the ID's for the LVM paritions*
vi /etc/fstab
*replace the ID's of the old /, /boot, ...*
apt-get install lvm2 mdadm
mdadm --examine --scan >> /etc/mdadm.conf
*delete the previous lines starting with ARRAY, if any*
vi /etc/mdadm/mdadm.conf
*now update initramfs to make sure it contains MDADM and LVM2 support and install grub to the new disk*
update-initramfs -k all -u -v
grub-install /dev/sdz

At this point you should be able to shut down the src server and boot from the new disk after replacement (or if you use this to move a server by starting the dst server). If you are able to boot succesfully and have the second disk already in place we are now going to "restore" the mirror.

*copy the MBR*
dd if=/dev/sdz of=/dev/sdy bs=512 count=1
partprobe /dev/sdy
*add the disk to the MDADM RAID to start rebuilding*
mdadm --manage /dev/md0 --add /dev/sdy1
*check if the rebuild is started*
cat /proc/mdstat
*just to make sure reinstall grub on boot disks, choose both /dev/sdz en /dev/sdy*
dpkg-reconfigure grub-pc

Another reboot after the rebuild is finished can be good to verify everything.